The dog stop is a good shout actually. If your child carried one as he passed the gate and the dog came out, then he could blast the dog and the noise would be loud enough to allert everyone within a half mile radius. Perhaps the can could be stored near the entrance to the property?
I have to say as a out and out dog lover that this dog should be taken from these people immediately and if no home is available (RAF etc) then he should be put to sleep. I hate to say it because I feel that any dog can be rehabilitated given time. Unfortunately, the next child to get bitten will be too higher price to be paid.